Subject: [RFC PATCH v2] regmap: smbus: add support for regmap over SMBus

From: [email-masked] (Lars-Peter Clausen)
Date: Tue, 15 Apr 2014 15:08:27 +0200

On 04/15/2014 02:40 PM, Boris BREZILLON wrote:
>
> On 15/04/2014 14:25, Lars-Peter Clausen wrote:
>> On 04/15/2014 01:54 PM, Boris BREZILLON wrote:
>>>
>>>
>>> On 15/04/2014 12:09, Mark Brown wrote:
>>>> On Tue, Apr 15, 2014 at 09:36:13AM +0200, Boris BREZILLON wrote:
>>>>> On 14/04/2014 23:04, Mark Brown wrote:
>>>
>>>>>> The transfer type gets set once per device at init time so why not
>>>>>> just parameterise based on val_bytes?
>>>
>>>>> Actually, you may want to transfer 1 byte registers using the block
>>>>> method (if your device only support block transfers). This depends on
>>>>> the device being accessed and what it supports, but I'm not sure we
>>>>> can
>>>>> assume 1 byte registers will always be transferred using SMBUS byte
>>>>> transfers.
>>>
>>>> OK, so if this a realistic issue then it seems like it's better to
>>>> implement three different buses - there is not really any common code
>>>> between the various paths.
>>>
>>> Okay, I'll create 4 different busses (one for each access type).
>>> BTW, should I keep these implementations in the same source file
>>> (regmap-smbus.c) ?
>>> And, should I keep one method to register an smbus regmap or should I
>>> provide one method per access type and get rid of the
>>> regmap_smbus_transfer_type enum ?
>>
>> I don't think we should leave the decision which bus to use to the
>> driver. Neither should the driver have to choose whether to use smbus
>> or native I2C. We want to use native I2C when available, because it is
>> more efficient than going through the smbus emulation layer. On the
>> other hand we want to automatically switch to smbus when native I2C is
>> not available and the device can work fine with smbus. Also I'm afraid
>> that we'll otherwise soon see code popping up like:
>>
>> if (of_property_read_bool(np, "use-smbus-regmap"))
>>      regmap = regmap_init_smbus(...)
>> else
>>      regmap = regmap_init_i2c(...)
>>
>> My suggestion is that in regmap_init_i2c() you check the capabilities
>> of the I2C adapter. If it supports native I2C you setup the regmap
>> with the regmap_i2c struct just as it does right now. If the adapter
>> does not support native I2C, check if the device can be supported by
>> smbus (reg_bytes == 8 && val_bytes % 8 == 0). For each type of smbus
>> operations have one regmap_bus struct, and if you can fallback to
>> smbus choose the bus depending on the config's val_bytes.
>>
>
> What if the device only support SMBus block transfers, but the adapter
> support both regular I2C and SMBus block transfers (don't know if such a
> device exist :-)) ?

SMBus block transfers prefix the transfer with the number of bytes that are 
in this transfer. We do not support this currently in the I2C backend (but 
we could). If there should ever be a driver that needs support for SMBus 
block transfers this could easily be added. For now I think it is safe to 
ignore this.

>
> My point is that I'm not sure the core remap-i2c code can decide whether
> to use SMBus or I2C transfer only from val_bytes value, but I might be
> wrong.

It must be able to figure out what to do based on the config. If it is not 
we probably need to extend the config. E.g. if we should ever need to 
support devices which require SMBus block transfers we could add a flag that 
tells regmap that this device want's all messages prefixed with the length 
of the message.

- Lars